Multilevel mechanical grating device

ABSTRACT

A mechanical grating device for improving the diffraction efficiency. The mechanical grating device is built on a base having a surface. Above the base a spacer layer, having an upper surface, is provided, and a longitudinal channel is formed in said spacer layer, said channel having a first and second opposing side wall and a bottom. A plurality of spaced apart deformable ribbon elements are disposed parallel to each other. The deformable elements are organized in groups of N elements wherein N is greater than 2. When the device is actuated each of said groups forms a pattern of discrete levels wherein the pattern has n levels wherein n is greater than 2.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] Reference is made to U.S. Ser. No. 09/______ (EK Docket No.78,657), filed concurrently, entitled “A Mechanical Grating Device,” andto U.S. Serial No. 09/______ (EK Docket No. 78,593), also filedeconcurrently, entitled, “Method for Producing Co-planar SurfaceStructures.”

FIELD OF THE INVENTION

[0002] This invention relates to the field of modulation of an incidentlight beam by the use of a mechanical grating device. More particularly,this invention discloses a multilevel mechanical grating device whichhas a significant improvement in the output of the diffracted light beamby approximating a continuous blaze grating with m discrete levels.

BACKGROUND OF THE INVENTION

[0003] Electro-mechanical spatial light modulators have been designedfor a variety of applications, including image processing, display,optical computing and printing. Optical beam processing for printingwith deformable mirrors has been described by L. J. Hornbeck; see U.S.Pat. No. 4,596,992, issued Jun. 24, 1986, entitled “Linear Spatial LightModulator and Printer”. A device for optical beam modulation usingcantilever mechanical beams has also been disclosed; see U.S. Pat. No.4,492,435, issued Jan. 8, 1995 to Banton et al., entitled “MultipleArray Full Width Electro-mechanical Modulator,” and U.S. Pat. No.5,661,593, issued Aug. 26, 1997, to C. D. Engle entitled “LinearElectrostatic Modulator”. Other applications of electro-mechanicalgratings include wavelength division multiplexing and spectrometers; seeU.S. Pat. No. 5,757,536, issued May 26, 1998, to Ricco et al., entitled“Electrically-Programmable Diffraction Grating”.

[0004] Electro-mechanical gratings are well known in the patentliterature; see U.S. Pat. No. 4,011,009, issued Mar. 8, 1977 to Lama etal., entitled “Reflection Diffraction Grating Having a ControllableBlaze Angle”, and U.S. Pat. No. 5,115,344, issued May 19, 1992 to J. E.Jaskie, entitled “Tunable Diffraction Grating”. More recently, Bloom etal. described an apparatus and method of fabrication for a device foroptical beam modulation, known to one skilled in the art as agrating-light valve (GLV); see U.S. Pat. No. 5,311,360, issued May 10,1994, entitled “Method and Apparatus for Modulating a Light Beam”. Thisdevice was later described by Bloom et al. with changes in the structurethat included: 1) patterned raised areas beneath the ribbons to minimizecontact area to obviate stiction between the ribbon and substrate; 2) analternative device design in which the spacing between ribbons wasdecreased and alternate ribbons were actuated to produce good contrast;3) solid supports to fix alternate ribbons; and 4) an alternative devicedesign that produced a blazed grating by rotation of suspended surfaces;see U.S. Pat. No. 5,459,610, issued Oct. 17, 1995, to Bloom et al.,entitled “Deformable Grating Apparatus for Modulating a Light Beam andIncluding Means for Obviating Stiction Between Grating Elements andUnderlying Substrate,” and U.S. Pat. No. 5,808,797, issued Sep. 15, 1998to Bloom et al., entitled “Method and Apparatus for Modulating a LightBeam.” Bloom et al. also presented a method for fabricating the device;see U.S. Pat. No. 5,677,783, issued Oct. 14, 1997, entitled “Method ofMaking a Deformable Grating Apparatus for Modulating a Light Beam andIncluding Means for Obviating Stiction Between Grating Elements andUnderlying Substrate”.

[0005] The GLV device can have reflective coatings added to the topsurface of the ribbons to improve the diffraction efficiency andlifetime of the GLV device. Preferred methods of fabrication use siliconwafers as the substrate materials requiring the device to operate inreflection for the wavelengths of interest. An increase in reflectivityis important to reduce damage of the top surface of the ribbons andavoid mechanical effects that might be attributed to a significantincrease in the temperature of the device due to light absorption.

[0006] For GLV devices, the positions and heights of the ribbons havebeen symmetric in design. One drawback to this design is an inability toisolate the optical intensity into a single optical beam. Thisrelatively poor optical efficiency is primarily due to the symmetry ofthe actuated device, which produces pairs of equal intensity opticalbeams. Each period of the improved grating must include more than tworibbons and create an asymmetric pattern of the ribbon heights. Bycreating an asymmetric pattern for the heights of the ribbons, theintensity distribution of the diffracted optical beams is asymmetric andcan produce a primary beam with a higher optical intensity. Furthermore,by adjusting the asymmetry of the pattern of ribbon positions andheights, the intensity distribution of the diffracted optical beams canbe altered. In this way, the device can be used to switch betweenvarious diffracted optical beams.

SUMMARY OF THE INVENTION

[0007] It is an object of the present invention to provide a mechanicalgrating device wherein the diffraction efficiency of a blazed grating isaccomplished.

[0008] The object is achieved by a mechanical grating device comprising:

[0009] a base having a surface;

[0010] a spacer layer having an upper surface, is provided above thebase, and a longitudinal channel is formed in said spacer layer, saidchannel having a first and second opposing side wall and a bottom;

[0011] a plurality of spaced apart deformable ribbon elements disposedparallel to each other and spanning the channel, said deformable ribbonelements defining a top and a bottom surface and are fixed to the uppersurface of the spacer layer on each side of the channel, said deformableelements are organized in groups of N elements wherein N is greater than2; and

[0012] each of said groups forms a pattern of discrete levels in anactuated state wherein the pattern has n levels wherein n is greaterthan 2.

[0013] It is a further object of the present invention to provide anelectro-mechanical grating device wherein the diffraction efficiency ofa blazed grating is accomplished.

[0014] The object is achieved by an electro-mechanical grating devicecomprising:

[0015] a base having a surface;

[0016] a spacer layer, having an upper surface, is provided above thebase, and a longitudinal channel is formed in said spacer layer, saidchannel having a first and second opposing side wall and a bottom;

[0017] a first conductive layer being provided below the bottom of thechannel;

[0018] a plurality of spaced apart deformable ribbon elements disposedparallel to each other and spanning the channel, said deformable ribbonelements defining a top and a bottom surface and are fixed to the uppersurface of the spacer layer on each side of the channel, said deformableelements are organized in groups of N elements wherein N is greater than2;

[0019] each of said groups forms a pattern of discrete levels in anactuated state wherein the pattern has n levels wherein n is greaterthan 2; and

[0020] a second conductive layer being part of each actuable ribbonelement.

[0021] An advantage of the mechanical grating device of the invention isthat the position of the ribbons across the area of the substrate andthe periodic sequence of the ribbon heights can be used to improve thediffraction efficiency of the optical beam. This invention presents aperiodic sequence of ribbon heights that resembles a blazed grating withdiscrete levels and is predicted to significantly increase the opticaldiffraction efficiency. The multi-level mechanical grating device can befabricated using methods that are compatible with the microelectronicsindustry. The device is more reliable and more appropriate for printingapplications than other blazed mechanical and/or electro-mechanicalgratings in the patent literature. Further advantageous effects of thepresent invention are disclosed in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The subject matter of the invention is described with referenceto the embodiments shown in the drawings.

[0023]FIG. 1 is an illustration of diffraction from a binary reflectivegrating;

[0024]FIG. 2 is an illustration of diffraction from a blazed reflectivegrating;

[0025]FIG. 3 is an illustration of a multi-level diffraction grating toapproximate a blazed grating;

[0026]FIG. 4 is a perspective, partially cut-away view of the multilevelmechanical grating device of the present invention;

[0027]FIG. 5 is a top view of the multilevel mechanical grating deviceof the present invention;

[0028]FIG. 6 is a cross-sectional view along plane A-A indicated in FIG.5 to illustrate the layered structure of one embodiment of theinvention;

[0029]FIG. 7 is a cross-sectional view along plane B-B indicated in FIG.5 of the three level mechanical grating device wherein no force isapplied to the deformable ribbons;

[0030]FIG. 8 is a cross-sectional view along plane B-B indicated in FIG.5 of the three level mechanical grating device wherein force is appliedto the deformable ribbons;

[0031]FIG. 9 is a cross-sectional view along plane B-B indicated in FIG.5 of the four level mechanical grating device wherein no force isapplied to the deformable ribbons;

[0032]FIG. 10 is a cross-sectional view along plane B-B indicated inFIG. 5 of the four level mechanical grating device wherein force isapplied to the deformable ribbons; and

[0033]FIG. 11 shows the effect of gap width on diffraction efficiency ofa two-, three- and four-level grating light valve.

DETAILED DESCRIPTION OF THE INVENTION

[0034] Periodic corrugations on optical surfaces (i.e. diffractiongratings) are well known to perturb the directionality of incidentlight. Collimated light incident in air upon a grating is diffractedinto a number of different orders, as described by the grating equation,$\begin{matrix}{{{\sin \quad \theta_{m}} = {{\sin \quad \theta_{0}} + \frac{m\quad \lambda}{\Lambda}}},} & (1)\end{matrix}$

[0035] where λ is the wavelength of the light and m is an integerdenoting the diffracted order. FIG. 1 illustrates a reflective grating10 having an optical beam 12 incident on the grating 10 at an angleθ₀ 11with respect to an orthogonal axis O-O of the reflective grating 10. Thegrating surface is defined to have a period Λ 13, which defines theangles of diffraction according to the relation presented in Equation 1.A diffracted beam 16 corresponding to diffraction order m exits thegrating 10 at an angle θ_(m) 15.

[0036] The diffraction grating 10 pictured in FIG. 1 is a binary gratingwhere the grating profile is a square wave. The duty cycle is defined asthe ratio of the width of the groove L₁ 14 to the grating period Λ 13. Abinary phase grating will have the maximum diffraction efficiency whenthe duty cycle is equal to 0.5 and R, the reflectivity, is equal to 1.0.

[0037] For uniform reflectivity and 0.5 duty cycle, the relationpresented in Equation 2 is appropriate for the calculation of thetheoretical diffraction efficiency, within the accuracy of scalardiffraction theory. $\begin{matrix}{{\eta_{m} = {R\quad {\cos^{2}\left( {\frac{\pi}{\lambda}\left( {{q_{m}d} - {m\quad {\lambda/2}}} \right)} \right)}\frac{\sin^{2}\left( {m\quad {\pi/2}} \right)}{\left( {m\quad {\pi/2}} \right)^{2}}}},} & (2)\end{matrix}$

[0038] where q_(m) is a geometrical factor, $\begin{matrix}\begin{matrix}{q_{m} = {{\cos \quad \theta_{0}} + {\cos \quad \theta_{m}}}} \\{= {1 + {\sqrt{1 - \left( {m\quad {\lambda/\Lambda}} \right)^{2}}\quad {for}\quad {normal}\quad {incidence}\quad {\left( {\theta_{0} = 0} \right).}}}}\end{matrix} & (3)\end{matrix}$

[0039] For normally incident illumination, the maximum efficiency in thefirst (m=1) order occurs when the grating depth d=λ/4. Such a gratinghas equal diffraction efficiencies into the +1 and −1 orders ofapproximately 40% for the gratings of interest (λ/Λ≦0.5), while theremaining light is diffracted into higher odd orders (i.e. ±3, ±5,etc.).

[0040] For applications requiring a high optical throughput, the gratingis desired to diffract with a very high efficiency into a single opticalbeam. It is well known to one skilled in the art that this is bestaccomplished by shaping the grating profile with a blaze, see C. Palmer,ed., Diffraction Grating Handbook, 2^(nd) ed., (Milton Roy Instruments,Rochester, N.Y., 1993). FIG. 2 illustrates the continuous blazed gratingprofile 20 with a beam 22 incident on the plane of the grating surface24 to produce diffracted beams 26 a, 26 b, 26 c, 26 d, 26 e, 26 fassociated with the non-zero orders of diffraction. By proper design ofthe grating profile the intensity of the beam in the +1 diffracted order26 d is maximized.

[0041] The preferred methods of fabricating a grating device do notallow the grating profile illustrated in FIG. 2. FIG. 3 is anillustration of the grating profile that would be produced usingmicroelectronic fabrication techniques to approximate the blaze withdiscrete steps wherein each step represents a separate level. Thegrating profile 30 is a multilevel step grating that approximates acontinuous blazed grating profile 32 having a width L₂ 34 and a heightof separation h₂ 38. Equation 4 is the scalar diffraction theoryexpression for the efficiency of diffraction. The number of discretesteps N within this expression defines the grating profile 30. For theGLV device, the value of the integer is selected based on the period ofthe grating profile and the selected width of the ribbon L₂ 34. Thevalue of L₂ 34 is chosen to achieve the required diffraction efficiency,but is limited to a minimum by the available fabrication methods. Thevalue for the height h₂ 38 is optimized for maximum intensity in the +1diffracted optical  beam  according  to  the  expression  $h_{2} = {{{\lambda/2}N} + {p\frac{\lambda}{2}}}$where  p  is  0  or  a  positive  integer.

[0042] The diffraction efficiency η_(m) into the m^(th) order for agrating with N steps tuned to the +1 order is predicted via scalartheory to be, $\begin{matrix}{\eta_{m} = {\frac{R}{N^{2}}{{\sum\limits_{l = 0}^{N - 1}^{\quad \frac{\pi \quad l}{N}{({q_{m} - {2m}})}}}}^{2}{\frac{\sin^{2}\left( {m\quad {\pi/N}} \right)}{\left( {m\quad {\pi/N}} \right)^{2}}.}}} & (4)\end{matrix}$

[0043] As an example of using these relationships, Table 1 shows thediffraction efficiency into the −3 through +3 orders for gratings withdiffering discrete steps N and R (reflectivity) equal to 1.0. With theaddition of a third discrete step, the grating profile becomesasymmetric and the intensity in the +1 diffracted beams 26 d isincreased by 70% over the power obtained for a grating profile having asquare wave profile, N=2. The improvement in diffraction efficiencyincreases with an increasing number of step levels N. TABLE 1 N η⁻³ η⁻²η⁻¹ η₀ η₁ η₂ η₃ 2 0.045 0 0.405 0 0.405 0 0.045 3 0 0.171 0 0 0.684 0 04 0.090 0 0 0 0.811 0 0 5 0 0 0 0 0.875 0 0

[0044] For the application of the device described here to printing byphotosensitive media or thermal sensitive methods, the efficiency shouldbe maximized to allow faster rates of printing while reducing the powerrequirements of the optical sources providing the incident illumination.For display and other applications, increased efficiency is alsoadvantageous. Ideally, the continuous blaze grating profile could beused to maximize the efficiency of a single diffracted order. Because ofthe fabrication methods chosen, the alternative of using multiple steplevels is desirable. FIG. 3 illustrates a grating profile that can beproduced using the standard fabrication processes of microelectronicdevices.

[0045] Referring now to FIG. 4 which illustrates a perspective,partially cut-away view of the multilevel mechanical grating device 100of the present invention. The multilevel mechanical grating device 100disclosed therein can form at least three different levels. Themechanically deformable structures of the device 100 are formed on topof a base 50. The present embodiment as shown in FIG. 4 discloses adevice 100 that can be operated by the application of an electrostaticforce. Because the actuation force of the multilevel mechanical gratingdevice 100 is electrostatic, the base 50 comprises several layers ofdifferent materials. The base 50 comprises a substrate 52 chosen fromthe materials glass and silicon, which is covered by a bottom conductivelayer 56. In this embodiment the thin bottom conductive layer 56 isnecessary since it acts as an electrode for applying the voltage toactuate the mechanical grating device 100. The thin bottom conductivelayer 56 is covered by a protective layer 58. The bottom conductivelayer 56 is selected from the group consisting of aluminum, titanium,gold, silver, tungsten, silicon alloys and indium tin oxide. Above theprotective layer 58 a standoff layer 60 is formed which is followed by aspacer layer 65. On top of the spacer layer 65, a ribbon layer 70 isformed which is covered by a reflective layer or layers 78. Thethickness and tensile stress of the ribbon layer 70 are chosen tooptimize performance by influencing the electrostatic or mechanic forcerequired for actuation and the returning force, which affects the speed,resonance frequency, and voltage requirements of the multilevelmechanical grating device 100. In the present embodiment the reflectivelayer 78 also has to include a conductor in order to provide electrodesfor the actuation of the multilevel mechanical grating device 100. Theelectrodes are patterned from the reflective and conductive layer 78.

[0046] The spacer layer 65 has a longitudinal channel 67 formed thereinthat extends along the longitudinal direction L-L of the multilevelmechanical gating device 100. The longitudinal channel 67 comprises afirst and second side wall 67 a and 67 b and a bottom 67 c. The channel67 is open on top and covered by a first and second set of deformableribbon elements 72 a and 72 b. Each deformable ribbon element 72 a and72 b spans the channel 67 and is secured to the surface of the spacerlayer 65 on either side of the channel 67. The bottom 67 c of thechannel 67 is covered by the protective layer 58. As mentioned above,the ribbon layer 70 is covered by the reflective layer 78. Thereflective layer 78 (conductive) is patterned such that there are firstand second conducting regions 78 a and 78 b, which form comb-likestructures arranged on the surface of the multilevel mechanical gratingdevice 100 in an interdigitated manner. The first and second conductiveregion 78 a and 78 b are mechanically and electrically isolated from oneanother. According to the pattern of the reflective layer 78, the ribbonlayer 70 is patterned to form the first and the second set of deformableribbon elements 72 a and 72 b spanning the channel 67. The deformableribbon elements 72 a and 72 b are grouped according to the longitudinaldirection L-L of the channel 67. In the case of the three levelmechanical grating device (embodiment as disclosed in FIG. 4) threedeformable ribbon elements belong to one group. Each group comprises onedeformable ribbon element from the second set 72 b and two deformableribbon elements from the first set 72 a.

[0047] In the embodiment shown in FIG. 4, a plurality of standoffs 61 ispositioned on the bottom 67 c of the channel 67. The standoffs 61 arepatterned from the standoff layer such that a group of standoffs 61 isassociated with the deformable ribbon elements 72 a and 72 b of eachgroup. In the embodiment shown here, the group of standoffs 61 isassociated with the second ribbon element 72 _(L3) a ₂of each group(valid for three ribbon elements per group). As shown in FIG. 7, eachgroup comprises a first, second and third ribbon element 72 _(L3) a ₁,72 _(L3) a ₂, and 72 _(L3) b ₁. The standoffs 61 may also be patternedin the form of a single bar.

[0048] A top view of the multilevel mechanical grating device 100 withthree levels is illustrated in FIG. 5, which also shows two planesperpendicular to the view illustrated. View Plane A-A is a side view ofthe multilevel mechanical grating device 100 and depicts the view shownin FIG. 6. View Plane B-B is a side view of the device and depicts theview shown in FIG. 7. Note that a device with four or more levels (fouror more deformable ribbon elements per group) is a straightforwardextension of the principles illustrated in FIGS. 5, 6 and 7.

[0049] The mechanical grating device 100 as shown in FIG. 5, is a devicewhich can be actuated by the application of an electrostatic force. Itis clear that a person skilled in the art can imagine other ways foractuating the grating device, for example thermal actuation,piezoelectric actuation or any combination. In the embodiment shown inFIG. 5, a first and a second electrically conducting region 78 a and 78b are formed on the surface of the mechanical grating device 100. Thefirst and the second electrically conducting region 78 a and 78 b areelectrically and mechanically isolated from each other to allow theapplication of different voltages to the first and second sets ofdeformable ribbon elements 72 a and 72 b. The first conducting region 78a applies the voltage to the first set of deformable ribbon elements 72a and the second conducting region 78 b provides the voltage to thesecond set of deformable ribbon elements 72 b. The second conductingregion 78 b is in contact with the bottom conductive layer 56 (see FIG.6) designated at the base 50 through at least one etched opening 74filled with the thick conducting layer 76. For operation of the device,the electrostatic force is produced by a voltage difference between thebottom conductive layer 56 and the conducting layer 78 atop the ribbonlayer 70. Ideally the conducting layer 78 is highly reflective tomaximize the optical diffraction efficiency when operating the device.The connection with the bottom conductive layer 56 is carried out by aninterconnect 75. The thin bottom conductive layer 56 is formed below thebottom 67 c of the channel 67. From the view of FIG. 5, regions of thespacer layer 65 and protective layer 58 are visible because ofpatterning of first and second conductive region 78 a and 78 b toachieve electrical and mechanical isolation of the deformable ribbonelements 72 a and 72 b.

[0050] The device presented here is a GLV that incorporates multiplelevels, which means more than two, to discretely approximate a blazedgrating. FIGS. 7 and 8 illustrate this concept with three levels, andFIGS. 9 and 10 illustrate the concept with four levels.

[0051] In FIG. 7, the surface 53 a of the substrate is shown withpedestals or lines as standoffs 61 designed with specific heights asdefined by the relationship between the height h₂ 34 and the number ofribbons N per group. For this case, the value of N is three for thegroup which represents one period Λ. The first ribbon element of eachgroup is designated 72 _(L3) a ₁, the second ribbon element of eachgroup is designated 72 _(L3) a ₂ and the third ribbon element of eachgroup is designated 72 _(L3) b ₁. The first and second ribbon element 72_(L3) a ₁ and 72 _(L3) a ₂ of each group are contacted by the firstconductive region 78 a or, in other words, the first and second ribbonelements 72 _(L3) a ₁ and 72 _(L3) a ₂ of each group belong to the firstset of deformable ribbon elements 72 a. The third ribbon element 72_(L3) b ₁ of each group is contacted by the second conductive region 78b or the third ribbon element 72 _(L3) b ₁ of each group belongs to thesecond set of deformable ribbon elements 72 b. The height of theintermediate level is defined by standoff 61 which is associated withthe second ribbon element 72 _(L3) a ₂ of each group. In the unactuatedstate (no applied force) all the ribbon elements 72 a and 72 b arecoplanar, defining a first top level 64 b and a first bottom level 64 a.The unactuated multilevel mechanical grating device 100 acts like amirror and an incident light beam 90, having a wave-length λ, isreflected into the 0^(th) order. The reflected light beam in the 0^(th)order is designated 92 a. In the actuated state (FIG. 8) the deformableribbon elements 72 a of the first set are subjected to a deformationwhich draws the ribbon elements into the channel 67. The ribbon elements72 b of the second set are not subjected to any deformation. Thereforeevery third ribbon element 72 _(L3) b ₁ of each group remains in theunactuated state thereby defining the first top level 64 b and the firstbottom level 64 a. The second ribbon element of each group abuts againstthe standoff 61, thereby defining a first intermediate top level 54 b.The first element 72 _(L3) a ₁ of each group is moved to the bottom ofthe channel 67, defined by surface 53 a, thereby defining a bottom toplevel 53 b. Each top level 64 b, 54 b and 53 b is spaced by λ/2N abovethe surface 53 a to maximize the efficiency of diffraction into the +1order. The diffracted beam is designated 92 b.

[0052] Although the ribbons in each group are actuated to differentdepths, each does not have to be independently addressed by the drivercircuitry. The presence of standoffs to define the height 54 a enablesthe device to operate as designed with all moving ribbons receiving thesame voltage and initial electrostatic force. Thus, only two independentvoltage levels are required to operate a device with improvedefficiency, ground voltage and operating voltage. This is equivalent tothe requirement of the device designs of prior art.

[0053] In FIGS. 9 and 10, in which N=4, the lower standoff height 61 isλ/8 and the upper standoff height 62 is λ/4. The total depth of thechannel should be (1−1/N)λ/2. For this case, the value of N is four fora group which represents one period Λ. The first ribbon element of eachgroup is designated 72 _(L4) a ₁, the second ribbon element of eachgroup is designated 72 _(L4) a ₂, the third ribbon element of each groupis designated 72 _(L4) a ₃ and the fourth ribbon element of each groupis designated 72 _(L4) b ₁. The heights of the intermediate levels aredefined by standoffs 61 which are associated with the second and thirdribbon element 72 _(L4) a ₂ and 72 _(L4) a ₃ of each group. The standoff61 associated with the second ribbon element 72 _(L4) a ₂ defines asurface 54 a. The standoff 61 associated with the third ribbon element72 a ₃ defines a surface 55 a. In the unactuated state (no appliedforce) all the ribbon elements 72 a and 72 b are coplanar, defining afirst top level 64 b and a first bottom level 64 a. The unactuatedmultilevel mechanical grating device 100 acts like a mirror and anincident light beam 90, having a wavelength λ, is reflected into the0^(th) order. The reflected light beam in the 0^(th) order is designated92 a. In the actuated state (FIG. 10) the deformable ribbon elements 72a of the first set are subjected to a deformation which draws the ribbonelements into the channel 67. The ribbon elements 72 b of the second setare not subjected to any deformation. Therefore every forth ribbonelement 72 _(L4) b ₁ of each group remains in the unactuated statethereby defining the first top level 64 b and the first bottom level 64a. The third ribbon element 72 _(L4) a ₃ of each group abuts against thestandoff 61, defining the surface 55 a, thereby defining a firstintermediate top level 55 b. The second ribbon element 72 _(L4) a ₂ ofeach group abuts against the standoff 61, defining the surface 54 a,thereby defining a second intermediate top level 55 b. The first element72 _(L4) a ₁ of each group is moved to the bottom of the channel 67,defined by surface 53 a, thereby defining a bottom top level 53 b. Eachtop level 64 b, 55 b, 54 b and 53 b is spaced by λ/2N above the surface53 a to maximize the efficiency of diffraction into the +1 order. Thediffracted beam is designated 92 b.

[0054] As discussed previously, the optical efficiency of the device cantheoretically be increased by up to 70% for a 3-level grating or 102%for a 4-level grating, assuming ideal reflectors and ignoring effectsfrom inter-ribbon gaps. Note that, while more levels yields higherdiffraction efficiencies in the ideal grating, the presence of gapsbetween ribbons degrades the performance of 3- and 4-level gratingsrelative to that of 2-level gratings. Furthermore, the additional levelswill increase the number of processing steps required to create thestandoffs 61. FIG. 11 shows a plot of the theoretical diffractionefficiency of the 1^(st)-order beam as a function of the percent ratioof gap width L_(G) to the ribbon width L_(R), within the accuracy ofscalar diffraction theory. In practice, with an optimized device, theratio L_(G)/L_(R) can be between 10% and 30% and the corresponding 3-and 4-level gratings still provide a significant improvement indiffraction efficiency. Thus, the ideal number of ribbons per period, N,is probably either three or four, depending on the minimum feasible sizeof the gaps between the ribbons and the allowed pixel width.

[0055] The invention has been described in detail with particularreference to certain preferred embodiments thereof, but it will beunderstood that variations and modifications can be effected within thespirit and scope of the invention.

PARTS LIST

[0056]10 reflective grating

[0057]11 angle θ₀

[0058]12 optical beam

[0059]13 period Λ

[0060]14 width of the groove

[0061]15 angle θ_(m)

[0062]16 diffracted beam

[0063]20 blazed grating

[0064]22 incident beam

[0065]24 grating surface

[0066]26 a to 26 f diffracted beams

[0067]30 grating profile

[0068]32 continuous blazed grating profile

[0069]34 width L₂

[0070]38 a height of separation h₂

[0071]50 base

[0072]50 a top surface of base

[0073]52 substrate

[0074]53 surface of the base

[0075]53 a surface

[0076]53 b

[0077]54 a top surface of standoffs

[0078]54 b second intermediate top level

[0079]55 a surface

[0080]55 b first intermediate top level

[0081]56 bottom conductive layer

[0082]58 protective layer

[0083]60 standoff layer

[0084]61 standoff

[0085]64 a first bottom level

[0086]64 b first top level

[0087]65 spacer layer

[0088]66 sacrificial layer

[0089]67 channel

[0090]67 a first side wall of the channel

[0091]67 b second side wall of the channel

[0092]67 c bottom of the channel

[0093]70 ribbon layer

[0094]70 a bottom surface of the coplanar ribbon elements

[0095]70 b top surface of the coplanar ribbon elements

[0096]72 a first set of deformable ribbon elements

[0097]72 b second set of deformable ribbon elements

[0098]72 _(L3) a ₁ first element of each group of three

[0099]72 _(L3) a ₂ second element of each group of three

[0100]72 _(L3) b ₁ third element of each group of three

[0101]72 _(L4) a ₁ first element of each group of four

[0102]72 _(L4) a ₂ second element of each group of four

[0103]72 _(L4) a ₃ third element of each group of four

[0104]72 _(L4) b ₁ fourth ribbon element of each group of four

[0105]74 opening

[0106]75 interconnect

[0107]76 thick conductor

[0108]78 a first conducting region

[0109]78 b second conducting region

[0110]92 b diffracted beam

[0111]100 multilevel mechanical grating device

[0112] L longitudinal direction

[0113] N number of discrete steps

[0114] d grating depth

[0115] m order

[0116] n number of levels

[0117] η_(m) diffraction efficiency

[0118] A-A view plane

[0119] B-B view plane

[0120] L-L longitudinal direction of the device

[0121] O-O orthogonal axis

What is claimed is:
 1. A mechanical grating device comprising: a basehaving a surface; a spacer layer, having an upper surface, is providedabove the base, and a longitudinal channel is formed in said spacerlayer, said channel having a first and second opposing side wall and abottom; a plurality of spaced apart deformable ribbon elements disposedparallel to each other and spanning the channel, said deformable ribbonelements defining a top and a bottom surface and are fixed to the uppersurface of the spacer layer on each side of the channel, said deformableelements are organized in groups of N elements wherein N is greater than2; and each of said groups forms a pattern of discrete levels in anactuated state wherein the pattern has n levels wherein n is greaterthan
 2. 2. The mechanical grating device as recited in claim 1 has aplurality of standoffs provided, and according to the longitudinaldirection of the device at least N-2 standoffs are associated with eachgroup.
 3. The mechanical grating device as recited in claim 2 whereinthe standoffs are formed on the bottom of the channel.
 4. The mechanicalgrating device as recited in claim 2 wherein the standoffs are formed onthe bottom surface of the ribbon elements.
 5. The mechanical gratingdevice as recited in claim 2 wherein according to the width of saiddevice each standoff is divided into a plurality of individual elementsof equal height.
 6. The mechanical grating device as recited in claim 1wherein in the actuated state the levels of adjacent ribbon elements ineach group are separated by$\frac{\lambda}{2N} + {p{\frac{\lambda}{2}.}}$


7. The mechanical grating device as recited in claim 6 wherein in theactuated state the levels of successive ribbon elements in each groupare reduced by a constant amount with respect to the bottom of thechannel, and thereby representing a staircase of equal steps.
 8. Themechanical grating device as recited in claim 1 wherein said side wallsare substantially vertically disposed with respect to the bottom.
 9. Themechanical grating device as recited in claim 1 wherein said channel hasa constant cross section along the entire length of the device.
 10. Themechanical grating device as recited in claim 1 wherein a reflectivelayer is provided on the top surface of the ribbon elements.
 11. Anelectro-mechanical grating device comprising: a base having a surface; aspacer layer, having an upper surface, is provided above the base, and alongitudinal channel is formed in said spacer layer, said channel havinga first and second opposing side wall and a bottom; a first conductivelayer being provided below the bottom of the channel; a plurality ofspaced apart deformable ribbon elements disposed parallel to each otherand spanning the channel, said deformable ribbon elements defining a topand a bottom surface and are fixed to the upper surface of the spacerlayer on each side of the channel, said deformable elements areorganized in groups of N elements wherein N is greater than 2; each ofsaid groups forms a pattern of discrete levels in an actuated statewherein the pattern has n levels wherein n is greater than 2; and asecond conductive layer being part of each actuable ribbon element. 12.The electro-mechanical grating device as recited in claim 11 has aplurality of standoffs provided, and according to the longitudinaldirection of the device at least N-2 standoffs are associated with eachgroup.
 13. The electro-mechanical grating device as recited in claim 12wherein the standoffs are formed on the bottom of the channel.
 14. Theelectro-mechanical grating device as recited in claim 12 wherein thestandoffs are formed on the bottom surface of the ribbon elements. 15.The electro-mechanical grating device as recited in claim 12 whereinaccording to the width of said device each standoff is divided into aplurality of individual elements of equal height.
 16. Theelectro-mechanical grating device as recited in claim 11 wherein in theactuated state the levels of adjacent ribbon elements in each group areseparated by $\frac{\lambda}{2N} + {p{\frac{\lambda}{2}.}}$


17. The electro-mechanical grating device as recited in claim 16 whereinin the actuated state the levels of successive ribbon elements in eachgroup with respect to the bottom of the channel are reduced by aconstant amount, thereby representing a staircase of equal steps. 18.The electro-mechanical grating device as recited in claim 11 whereinsaid side walls are substantially vertically disposed with respect tothe bottom.
 19. The electro-mechanical grating device as recited inclaim 11 wherein said channel has a constant cross section along theentire length of the device.
 20. The electro-mechanical grating deviceas recited in claim 11 wherein a reflective layer is provided on the topsurface of the ribbon elements.
 21. The electro-mechanical gratingdevice as recited in claim 11 comprises a driving means for applying avoltage between the first and the second conductive layer to actuate theribbon elements.